Increasingly, the capacity and reliability of communication systems is a focus driving much of systems technology. Employing multiple-input, multiple-output (MIMO) communication systems is an area that supports this growth in the development of wireless networks. MIMO communication systems have been shown to provide improvements in both capacity and reliability over single-input, single-output (SISO) communication systems. These MIMO communication systems commonly employ a block structure wherein a MIMO transmitter (which is a cooperating collection of N single-dimension transmitters) sends a vector of symbol information. This symbol vector may represent one or more coded or uncoded SISO data symbols. A MIMO receiver (which is a cooperating collection of M single-dimension receivers, M≧N) receives one or more copies of this transmitted vector of symbol information. The performance of the entire communication system hinges on the ability of the receiver to find reliable estimates of the symbol vector that was transmitted by the transmitter. This necessitates that the MIMO receiver provide reliable channel estimates associated with transmissions from the MIMO transmitter.
For example, a 2×2 MIMO communication system may transmit two independent and concurrent signals, employing two single-dimension transmit antennas and two single-dimension receive antennas. Alternatively, the antennas could be derived from a single physical antenna that appropriately employs polarization. Two receive signals Y1(k), Y2(k) on the kth sub-carrier/tone following a Fast Fourier Transformation and assuming negligible inter-symbol interference may be written as:Y1(k)=H11(k)*X1(k)+H12(k)*X2(k)+n1(k)Y2(k)=H21(k)*X1(k)+H22(k)*X2(k)+n2(k)where X1(k) and X2(k) are two independent signals transmitted on the kth sub-carrier/tone from the first and second transmit antennas, respectively, and n1 and n2 are noises associated with the two receive signals. The term Hij(k), where i=1, 2 and j=1, 2, incorporates gain and phase distortion associated with symbols transmitted on the kth sub-carrier/tone from transmit antenna j to receive antenna i. The channel gain and phase terms Hij(k) may also include gain and phase distortions due to signal conditioning stages such as filters and other analog electronics. The receiver is required to estimate the channel values Hij(k) to reliably decode the transmitted signals X1(k) and X2(k).
To estimate the channel coefficients Hij(k) at the receiver, the transmitter and the receiver employ training sequences. These training sequences are predetermined and known at both the transmitter and the receiver. In an IEEE 802.11(a) compliant system, a training sequence (called a long sequence) is employed as part of a preamble to the transmission of data. This long sequence involves the transmission of a known sequence of vector symbols, employing 52 excited tones (1 or −1), an unexcited tone (0) at DC and unexcited tones at each end of the spectrum, to provide guard bands that are used to protect data tones from pass band filter effects.
Established channel estimates may be subject to depreciating influences during data transmission due to errors in sampling clocks and carrier frequencies associated with the transmitting and receiving systems. A common problem is a phase error associated with the data transmission, which degrades the integrity of the existing channel estimates and the resulting decoder performance of the receiver.
Accordingly, what is needed in the art is a way to improve established channel estimates for data transmissions associated with a MIMO communication system.